The present invention relates to computer systems and more particularly to systems for controlling and selecting the colors used in graphic images generated by computer systems.
The accurate control and specification of color in computer controlled color display devices is of great concern to those who utilize color as a means of organizing and displaying information. The desire to meet these control and specification requirements has resulted in the development of interfaces which allow the user to select different colors within the gamut of colors that can be produced by the display device. Such interfaces have in the past simply provided the user with the ability to specify digital-to-analog converter (DAC) values which correspond to voltages to be applied to the electron guns of color cathode-ray tube (CRT) displays. From a conceptual point of view, the interface allows the user to select the spatial coordinates corresponding to different colors through the use of a video red-green-blue (RGB) color space representing the signals applied to the display device. Prior color selection interfacers have not systematized the color selection process so as to provide an orderly approach to color selection or provided useful visual aids to assist in the color selection process.
The video RGB signal space color selection system has become widely used because it is readily correlated to the hardware (electron guns and associated drive circuitry) employed for creating CRT displays. However, it is important to note that the video RGB space is not a perceptually uniform color space. That is, at various locations within the space, a uniform change in the RGB designation will not necessarily result in a uniform change in the perceived color. The perceptual nonuniformity of the RGB space is a result of the nonlinearity of human vision in perceiving the color spectrum. The effect of the perceptual nonuniformity of the video RGB color space is that it is difficult for the user to predict what color will appear for any given change in RGB values.
In the past, numerous efforts have been made to develop perceptually uniform color spaces for facilitating color specification tasks. Many efforts to develop perceptually uniform color spaces have also been directed to correlating the color spaces to internationally accepted standards for color measurement so that the color can be accurately communicated and consistently reproduced. The most prominent international standards for color measurement are collectively termed the CIE system (Commission International de l'Eclairage or International Commission on Illumination).
The CIE system is based on the premise that specific perceived colors result from the proper combination of an illuminant or reference light source, an object, and an observer. A useful explanation of the CIE system is provided in "Principles of Color Technology", 2nd ed. 1981, by Billmeyer & Saltzman. Generally, the CIE system defines standard light sources having characteristic spectral power distribution curves. Each of the curves is a depiction of the relative luminous power of the source and the amount of light emitted by the source at each wavelength of the visible spectrum. The CIE system also defines a "standard observer" in terms of three color matching functions. The color matching functions are the relative magnitudes of three standard stimuli necessary to produce any color. Any object, the color of which is to be specified, has a characteristic spectral reflectance curve. The reflectance curve is a representation of the fraction of the light reflected from the object at each wavelength. The product of the spectral power distribution curve for a standard source and the reflectance curve of the object under study, when separately multiplied by each color matching function will, after suitable normalization, yield three curves, the area under each curve corresponding respectively to the three CIE tristimulus values XYZ. The values of the standard stimuli that define the color matching functions are such that the color matching function corresponding to the Y tristimulus value represents the human eye response to the total power of the light (i.e. luminance) reaching the eye. Accordingly, the tristimulus value Y provides an indication of the luminance of the color.
The CIE tristimulus values are related to a two-dimensional map of colors known as the 1931 CIE chromaticity diagram. The 1931 CIE diagram includes a horseshoe-shaped spectrum locus on which the spectral colors may be identified by their wavelengths. The coordinates of the chromaticity diagram are known as chromaticity coordinates x and y, and are derived by taking the ratios of the respective X and Y tristimulus values to the sum of all three tristimulus values X, Y and Z. The x and y chromaticity coordinates for any real color are located within the bounds of the spectrum locus and the line that joins the ends of the spectrum locus.
The x and y coordinates do not completely describe a color because they contain no information on the inherent luminance of a color. As noted, the Y component of the tristimulus values is a measure of the luminance of the color. Accordingly, a three-dimensional color specification system may be created by adding a third axis to the 1931 diagram which extends upwardly from the xy plane at the x and y coordinates of the source light. The third axis is the Y axis and is scaled in units of luminance. However, it is conventional to normalize the Y values from 0 to 1, representing the full range from black to white, respectively. At each level of luminance the area of the 1931 diagram, which represents the range of all possible colors that can occur, becomes smaller for increasing values of Y and terminates at a single "white point" at the maximum Y value.
The three-dimensional color specification system just described is known as the CIExyY system. In view of the above, it can be appreciated that any real color can be specified in terms of the CIExyY color specification system and directly related to the particular CIE tristimulus values XYZ. The CIExyY system is a widely accepted method for specifying color. Further, the 1931 CIE diagram or, more typically, data derived therefrom, is valuable because it can be used to predict the color of additive mixtures of two or more colors. That is, tristimulus values of component colors mathematically add to yield the tristimulus values of the resulting mixed color.
Efforts have been made to transform the CIE color specification system into a perceptually uniform color space, while preserving the additive mixing feature of the 1931 CIE chromaticity diagram.
One such transformation of the 1931 diagram includes a two dimensional uniform chromaticity diagram (known as the 1976 UCS diagram) having u' and v' coordinates that approximate a perceptually uniform color plane. The coordinates are known as the uniform chromaticity coordinates and are directly related to the x and y chromaticity coordinates (and hence, to the XYZ tristimulus values) as follows: EQU u'=4x/(-2x+12y+3)=4X/(X+15Y+3Z) (1) EQU v'=9y/(-2x+12y+3)=9Y/(X+15Y+3Z) (2)
As described, in the referenced text by Billmeyer & Saltzman, the 1976 UCS diagram defined by the u' and v' coordinates has been mathematically converted into a color space that approaches perceptual uniformity and is known as the CIELUV color space.
The u*,v* coordinates of the CIELUV system were formed with the achromatic colors at the origin (u*=0,v*=0) by subtracting the uniform chromaticity values u'.sub.n and v'.sub.n for the source light from those of the selected color.
The third coordinate of the CIELUV space, L*, known as the metric lightness function, lies perpendicular to the u*v* plane and intersects that plane at the origin. The L* axis is the axis of the achromatic colors (black, grey and white) and denotes variations in the lightness from L*=0 (black) to L*=100 (white).
As noted, all of the coordinates of the CIELUV space are directly related, via the CIExyY system to the CIE tristimulus values. These relationships are defined below: EQU L*=116 (Y/Y.sub.n).sup.1/3 -16; for (3)
Y/Y.sub.n greater than 0.008856 EQU L*=903.3 (Y/Y.sub.n); for Y/Y.sub.n ( 4)
less than or equal to 0.008856
where
Y=tristimulus value (lightness) of a color, and PA1 Y.sub.n =lightness of the reference light source EQU u*=13 L*(u'-u'.sub.n) (5) EQU v*=13 L*(v'-v'.sub.n) (6) PA1 u'.sub.n and v'.sub.n are the uniform chromaticity coordinates for the reference light source.
where
The modified cube-root function for L* as shown above, yields a perceptually uniform scaling of lightness. It is common to alternatively refer to the visual sensation of lightness as value.
Hue is defined in the CIELUV color space as the angle made relative to the positive u* axis. The hue angle, h*, is defined as follows: EQU h*=arctan (v*/u*) (7)
A third parameter, known as psychometric chroma C*.sub.uv, is adopted in conjunction with the CIELUV color space as a numerical representation of the chroma of a color. Chroma describes the saturation or vibrancy of a color. Chroma C*.sub.uv equates to distance from the L* axis at a particular level of lightness or value. Accordingly, the notation C*.sub.uv relates to the u*, v* coordinates, as follows: EQU C*.sub.uv =(u*.sup.2 +v*.sup.2).sup.178 ( 8)
The CIELUV space is the most nearly perceptually uniform space developed thus far.